An Introduction to Organic Chemistry - Canvas

An Introduction to Organic Chemistry

Dr Sandy Wilkinson

[email protected]

Demonstrate an understanding of the basics of organic chemistry: nomenclature, isomerism and mechanisms

Demonstrate an understanding of the basic chemistry associated with the organic functional groups covered

Apply functional group chemistry to suggest basic organic reaction schemes

Module AimsBy the end of this module students should be able to:

J. Smith, “Organic Chemistry”, 5th edition (2017),McGraw-Hill

C. E. Housecroft and E. C. Constable, “Chemistry with Mastering Chemistry”, 4th Edition (2009), Prentice Hall

P. Atkins, L. Jones and L. Laverman, “Chemical Principles: The Quest for Insight”, 5th edition (2016), W. H.Freeman

M. Hornby and J. Peach, “Foundations of Organic Chemistry”, 1997, Oxford University Press

Texts

www.chemguide.co.uk

http://www.knockhardy.org.uk/sci.htm

Useful Websites

1

http://www.chemguide.co.uk/

http://www.knockhardy.org.uk/sci.htm

Organic Chemistry

Urea

It was once believed that organic material (things found in natural systems) could only be produced by living organisms, although many thinkers were convinced that organic and inorganic matter follow the same chemical rules.

Friedrich Wohler (1828) however showed that urea (a natural product) could be produced from two inorganic materials; lead cyanate and ammonium chloride. Wohler is now regarded a major pioneer in organic chemistry.

Organic chemistry is concerned with the structure, properties and reactions of ‘organic compounds’.

Organic compounds contain covalently bonded carbon atoms along with other elements such as hydrogen, oxygen, nitrogen and so on.

Organic compounds have many uses including:

Pharmaceuticals (drugs/medicines)Plastics and polymersFood additives (e.g. preservatives, flavours)AgrochemicalsFuel additives

Organic compounds are also involved in life processes.

2

1. Introduction to organic chemistry

Organic Compounds

Aspirin Ascorbic Acid (Vitamin C)

Ethylbenzene (‘Styrene’)

Cysteine (an Amino Acid)

HO

H H

CH3

H3C

H3C

CH3

CH3 H

Cholesterol

H H

Me H

H H

H Me

H H

Me H

H H

H Me

H H

Me H

H H

H Me

H H

Me H

H H

H Me

Polypropylene 3


Some Fundamental Concepts➢ Electrons occupy ‘orbitals’ which have different energies and

shapes.

1s2p

2py

x

z

2p2s

➢ Covalent bonds involve pairs of electrons.

C HH

H

H

➢ Elements have different electronegativities.

https://sciencenotes.org/electronegativity-definition-and-trend/

How easily an atom attracts a pair of electrons in a covalent bond.

H Cl + -

4


N HH

H

➢ Molecules can have lone ‘pairs’.

➢ Reactions involve the breaking and making of bonds.

Cl Cl C HH

H

H

+ H Cl C HCl

H

H

+

➢ Molecules occupy 3D space.

5


Carbon Chemistry

Carbon undergoes Catenation – the ability to bond with the same element to form a series:

Carbon forms strong covalent bonds with itself and many other elements (e.g. H, O, N, S, Cl)

Chains

Rings

Carbon Forms four covalent bonds which allows branching.

Carbon can also form multiple bonds with itself or other elements.

CH3

CH

CH2

CH3

CH3

6


Organic Compounds

The carbon ‘skeleton’. This is the

backbone. This will remain

unchanged in most reactions.

The functional group

which is the reactive

part of the molecule.

This will undergo

changes under the

correct conditions.

Organic Molecules

can have more than

one functional group.

7


Organic compounds have structures defined by how their

constituent atoms are connected to each other and arranged in

space.

Aldehyde -al RCHO CH3CHO ethanalCH

O

Ketone -one RCOR CH3COCH3 propanoneC

OC

C

Carboxylic Acid -oic acid RCOOH COH

OCH3COOH ethanoic acid

Ester -yl -oate RCOOR COR

OCH3COOCH3 methyl ethanoate

Amide -amide RCONH2 CNH2

OCH3CONH2 ethanamide

Acyl Chloride -oyl chloride RCOCl CCl

OCH3COCl ethanoyl chloride

Nitrile

Amine

Nitro

-nitrile

-amine

nitro-

RCN CH3CN ethanenitrile

CH3NH2 methylamine

nitromethane

C NH2

C N

RNH2

RNO2 C NO2CH3NO2

Alkane

Alkene

Alkyne

-ane

-ene

-yne

RH C C

C C

C C

C2H6 ethane

C2H4 ethene

C2H2 ethyne

Haloalkane halo- RX C X C2H5Cl chloroethane

Alcohol -ol ROH C OH C2H5OH ethanol

Ether ether ROR C O C C2H5OC2H5 diethyl ether

Some Functional Groups

Group Stem General Formula + Structure Example

8


Formulae

Empirical Formula - The simplest whole-number ratio of atoms. There is no link between the structure of a molecule and its empirical formula. Different molecules can have the same empirical formula.

Molecular Formula – Shows the all the atoms, and the number of each atom in a molecule for example C3H6O2. For organic compounds, these are written C, then H, then other elements in alphabetical order, for example C4H8BrCl.

Ethane

H

C

H

H

C

H

H HEmpirical Formula: CH3

Molecular Formula: C2H6

Determining Empirical Formulae from Percentage CompositionA compound was found to have a composition of 85.63 % C and 14.37 % H. What is its empirical formula?

C H

85.63 14.37

12.01 1.008

7.13 14.25

7.13 7.13

1 2

EF is CH2

Divide percentages by RAM

Divide all by lowest

Make sure ratio is in whole numbers

[Use the following RAMs: C = 12.01, H = 1.008]

9


A compound containing only C, H and O was found to have a composition of 40.00 % C and 6.71 % H. What is its empirical formula?

40.00 6.71

12.01 1.008

3.33 6.66

3.33 3.33

1 2

CH2O

C H O

53.29

16.00

3.33

3.33

1

The compound was found to have a molecular mass between 55-65, what is its molecular formula?

C2H4O2

83.26 16.38

12.01 1.008

6.93 16.25

6.93 6.93

1 2.34

C H

A compound was found to have a composition of 83.26 % C and 16.38 % H and a molecular mass of 86. What is its molecular formula?

EF = C3H7

Make sure ratio is in whole numbers – if not, multiply by odd numbers(x3, x5 etc.) each until it is..

3 7

MF = C6H14

[Use the following RAMs: C = 12.01, H = 1.008, O = 16.00]

10


Determining Empirical Formulae from Combustion Data (Hydrocarbons) Complete combustion of hydrocarbons produces CO2 and H2O only. If a hydrocarbon is fully combusted and the masses of CO2

and H2O formed are known, the empirical formula of the hydrocarbon can be worked out.

CXHY + excess O2 → X CO2 + (Y/2) H2O

C8H18 + 12 ½ O2 → 8CO2 + 9H2O

For example:

A hydrocarbon is combusted with an excess of oxygen yielding 211.2 g of CO2 and 129.6 g of H2O. Give the empirical formula of the hydrocarbon.

CO2 H2O

211.2 129.6

44.01 18.016

4.80 7.19

4.80 4.80

X = 1 Y/2 = 1.5

EF is CH3

Find moles of products -divide masses of products by their RMM

Divide all by lowest

Make sure ratio is in whole numbers

CXHY + excess O2 → x CO2 + (Y/2) H2O

Y = 3

11


Displayed Formula – These are a graphic representations of organic molecules that reveal their structure. They show all the atoms present and the bonds linking them.

Skeletal Formula – These are a shorthand way of representing organic molecules. They show the carbon skeleton as lines (each line representing a covalent bond) with the functional groups added to the skeleton.

Representing Organic CompoundsMolecular formulas (e.g. C3H6O) merely show the number of atoms in a compound, not how they are connected to each other. There are several ways that the structure of organic compounds can be represented.

These maybe simplified further, as long as the bonding is clear an unambiguous.

COH

OCH3 CH2

COH

OC2H5

Other Representations

3D Computer Models (e.g. ChemDraw)

Model kits, such as ‘MolyMod’ 12


Drawing Molecules in ‘3D’Drawings of molecules in 2 dimensions, such as displayed and skeletal formula show how atoms are connected, however there are some important features that are omitted by these diagrams. 3-D representations can include information about the shapes of molecules, bond angles as well as certain kinds of isomerism (see later).

‘ball and stick’ representation of methane

C-H bonds in the plane of the screenC-H bond behind the plane of the screen

C-H bond projecting in front of the plane of the screen

Symbols are used to represent bonds that are in, behind or in front of the plane of the paper (or screen).

‘normal’ bond – represents bonds in the plane

wedged bond – represents bonds in front of the plane

dashed bond – represents bonds behind the plane

draw the ‘central’ atom

add the two bonds in the plain

draw the bond projecting forward

add the bond behind the plain.

13


Naming Organic CompoundsThe naming system of organic compounds is an unambiguous code that describes they way that the atoms are connected to each other and arranged in space.

ChainsOrganic compounds are built upon carbon chains and the most fundamental part of the naming system deals with this. The names for some of the more common chains are shown below.

meth 1eth 2prop 3but 4pent 5hex 6hept 7oct 8

C CC

C CC C

C CC C C

C CC C C C

C CC C C C C

C CC C C C C C

CC

C

Chains - NumberingThe carbon atoms along a chain should be numbered so that the position of particular groups attached to the chain can be located. Chains are numbered from one end, and where there are substituents on the chain the numbering is done from the end so that the substituent is on the lowest numbered carbon atom.

C CC C C C

x

1

C2 3 4 5 6 7

C CC C C C

x

1

C234567

14

2. Naming organic compounds

Branches to Carbon ChainsWhere there are branches to chains it is important to identify the longest chain and then start to consider the branches. The branches are named according to the number of carbon atoms in their chains:

Single and Multiple Carbon-Carbon BondsCarbon atoms can have a total of four covalent bonds. It is therefore possible for carbon atoms to have multiple bonds with each other in chains and this is reflected in naming the chains.

CC CC CC

anSingle carbon-carbon bonds only

enContains a carbon-carbon double bond

ynContains a carbon-carbon triple bond

When there is a carbon-carbon multiple bond in a chain the numbering of the chain is done so that the start of the multiple bond is at the lowest number.

C CC C C C1

C2 3 4 5 6 7

C CC C C C1

C234567

C CC C C C1

C2 3 4 5 6 7

C

H

H

H

C

H

H

C

H

H

H

H

C

H

H

C

H

H

C

H

H

methyl

ethyl

propyl

C H3

C H3 C H2

C H3 C H2C H2

15


C CC C C C C

H H H H H H H

H H H H H H

HH

CH3

1 2 3 4 5 6 7 3-methylheptane

Position (3-)

Branch (methyl)

Length of longest chain (7 = hept)

Only single C-C bonds (ane)

Longest chain

Longest chain

C

C C C C

H

H

H H HHH

H H H H

H

H1

2

3 4 5 6

C

C

H

H

H

2-chloropentane

2-methylhex-2-ene

Functional Groups

Haloalkanes (R-F, R-Cl, R-Br, R-I) have halide atoms attached to the carbon chain. There can be more than one halogen on a particular chain. As usual, the numbering is such that the substituents are on the lowest possible numbered positions.

Cl Cl

Cl2,3-dichloropentane

When different halides are on a chain, the numbering is done to get the lowest possible numbers for the positions and then it is named so that the substituents are in alphabetical order.

Br

Cl

5 –bromo-2-chloroheptane16


Alcohols (R-OH) have names ending in –ol. Apart from methanol and ethanol, a number is used to denote where the alcohol is attached to the chain. OH

propan-2-ol

OH

OH

pentan-2,3-diol

OH

2-methylpentan-3-ol

Aldehydes (R-CHO) have names ending in –al. The carbon atom of the aldehyde group counts as a member of the chain. The carbon of the aldehyde is always counted as number 1. Numbers are only used if there are substituents or branches on the main chain.

O

H

propanal

O

H

2-methylpentanal

Ketones (RCOR’) have names ending in –one. A number is used to identify the position of the C=O (carbonyl group) on the chain.

O

pentan-2-one

Carboxylic Acids (RCOOH) have names ending in –oic acid. The carbon in the acid group is part of the chain and is always counted as 1 when numbering.

O

OH

propanoic acid 3-hydroxybutanoic acid

O

OH

OH

17


Esters (RCOOR’) are derivatives of acids where the H is replaced by an alkyl group. The naming of acid is therefore a combination of the alkyl group followed by a name derived from the acid (-oate)

O

O

ethyl butanoate

Acyl Chlorides (RCOCl) are derivatives of acids where the H is replaced by a chlorine. The names end in –oyl chloride. As with acids, the carbon in the –COCl functional group counts as part of the carbon chain and is always number 1.

O

Cl

3-methylbutanoyl chloride

2Primary Amines (RNH ) have the amine group (-NH ) attached to a carbon chain. If the chain is short (n=1 or 2) there is no ambiguity about where the group is so –amine is attached to the alkyl group name. For longer groups amino- is used along with a number.

2

NH2

NH2

ethylamine 2-aminopentane

Secondary and Tertiary Amines (RR’NH, RR’R’’N) have two or three alkyl groups attached to a nitrogen atom. There name reflects the alkyl groups and how many of them there are.

NH N

diethylamine trimethylamine18


Ethers (ROR’) These are related to alcohols but the H atom has been replaced by an alkyl group. One way to name them is to use the term ‘alkoxy-’ (e.g. methoxy-) as a substituent.

1-methoxypropane 1-ethoxy-2-methylpropane

Simpler ethers may be named by listing the name of each of the names of the groups (alphabetical order) attached to the oxygen atom followed by ‘ether’.

methyl propyl ether ethyl 2-methylpropyl ether

Amides (RCONH ) are carboxylic acid derivatives where the –OH of the acid group is replaced by a –NH2 group. There names end with –amide.

2

O

NH2

propanamide

Nitriles (RCN) are compounds that contain a carbon atom with a triple bond with nitrogen. Their names ends with –nitrile. The carbon counts as part of the chain when numbering.

N

propanenitrile 19


Cyclic Compounds can be formed by closing alkanes or alkenes into rings. The naming of cyclic compounds uses the convention for naming chains put is preceded with cyclo. When cyclic compounds have more than one substituent the numbering is done to get the lowest values.

cyclopentane cyclohexane cyclohexene

OH OH

OH

cyclohexanol Cyclohexane-1,2-diol*

If a small, simple chain is attached to the ring then the alkyl group is considered as the substituent.

If the attached chain is large, or complex then the ring may be named a substituent.

ethyl cyclohexane

5-cyclohexylpentanal20


*See section on alcohols later for the naming of diols.

Shapes of Organic MoleculesShapes of covalent molecules can be predicted by Valence Shell Electron Pair Repulsion (VSEPR) Theory. This assumes that pairs of electrons (bonding and lone pairs) will be positioned in space to achieve minimum repulsion, and therefore lowest energy.

For ‘saturated’ molecules carbon atoms are surrounded by 4 single bonds separated by angle of 109.5.

109.5

Tetrahedral

The tetrahedral shape is also found in saturated chains with angles very close to the ideal angle of 109.5.

21

3. Shapes of organic compounds

Shapes of CycloalkanesCycloalkanes are usually represented in a way that makes them look planar. If this was the case then the C-C-C bond angles within the rings would be as follows:

60o 90o 108o 120o 128.5o 135o

If the rings were planar then the only the 5 membered ring (cyclopentane) would have angles close to the angle that is most stable (109.5o).

With the exception of cyclopentane (3 membered) cyclic compounds are able to adopt the most stable non-planar shape.

60o ~88o ~109o~110o

22


H

H

H

H

H

H

Shapes of Organic Molecules with Double and Triple Bonds

120

120

120

180

120

Benzene

120

23


Hybridisation and Bonding

C6

12.01

1s 2s 2p2 2 2

2p

2s2p

2py

x

z

sp

py

x

z

+

p

Hybridisation Linus Pauling

sp3

sp3

sp3sp3

Hybridisation is the combining of atomic orbitals to produce hybrid orbitals. This does not change the total number of orbitals, it just changes their shape. One possibility is when an s orbital and three p orbitals hybridise. This forms 4 sp orbitals which are 109.5 apart (tetrahedral).

3

This kind of hybridisation explains the bonding that takes place in saturated compounds (single bonds).

valence electrons

24

4. Hybridisation and bonding

Bonding – sp Hybrid Orbitals3

H

H

H

HC

Hybrid orbitals can overlap with any atomic orbital of a different atom to form a bond. For example, in methane each of the four sp orbitals overlap with the 1s orbitals of four hydrogen atoms.

3

This kind of bonding, which involves head-on overlapping of orbitals is called 𝝈 bonding (sigma bonding).

sp Hybridisation2

sp

py

x

z

+

p

If hybridisation takes place between an s orbital and two p orbitals then three sp orbitals are formed. This leaves one of the p orbitals unchanged (remember, hybridisation always results in the same total number of orbitals).

2

yp

sp2

sp2

sp2

The three sp orbitals sit on a plane at 120 to each other, with the un-hybridised p orbital perpendicular to the plane.

2

120

25


Head–on overlaps between sp orbitals and orbitals of other atoms to form 𝜎 bonds. The example below shows this bonding in ethene.

2

The overlap between the sp orbitals of each carbon atom has produced a 𝜎 bond. Remember though that each carbon atom has an electron in an un-hybridised p orbital.

2

Some overlap of the p orbitals is possible above and below the plane formed by the 𝜎 bonds. This overlap creates a slightly weaker covalent bond called a 𝜋 bond (pi-bond).

H

C

H H

H

C𝜎

𝜋

The combination of the 𝜎 and 𝜋 bonds constitutes the double bond between the two carbon atoms. The overall structure of ethane is a planar (flat) molecule with bonds 120 apart.

120

120

120

H

CH H

H

C𝜎

26


180

Bonding – sp Hybrid Orbitals

If an s orbital hybridises with a single p orbital then two sp hybrid orbitals are formed. This leaves two p orbitals that are un-hybridised. An example of this explains the structure of ethyne. A 𝜎 bond and two 𝜋 bonds means that there is a triple bond between the two carbon atoms, and the molecule is linear.

H H

27


IsomerismIsomerism is the term that is used to describe molecules that have the same molecular formula but have different structures. This leads to them having different chemical or physical properties.

H H

H C O C H

H H

H H

H C C O H

H H

andand

Cl

Cl

Cl

Cl

Cl

Cl

and

and

and

Isomers may have dissimilar physical, chemical and biochemical activity.

28

5. Isomerism

Structural IsomerismStructural isomers are molecules that have identical molecular formulae but their atoms are connected together differently. One example of this is Chain Isomerism because of the possibility of branching of carbon chains. An example of this is C5H12, which has 3 isomers.

BP = 36.1C BP = 27.9C BP = 9.5C

Carbon chains (or rings) can have functional groups on different positions. This gives rise to the possibility of Positional Isomers (also known as Regio Isomers). Some examples are shown below.

29

5. Isomerism

It is important not to draw ‘false isomers’, these look different because of the way they are drawn rather than the way their atoms are connected.

Not isomers

Not isomers

An aldehyde A ketone

A carboxylic acid An ester

Another form of structural isomerism is Functional Group Isomerismwhere isomers have different functional groups (and hence often very different physical and chemical properties).

C H O3 6

C H O3 6 2

30

5. Isomerism

Stereoisomerism – Geometric IsomerismStereoisomers have their atoms joined in the same order but are arranged differently in space. One example of this is Geometrical Isomerism which occurs in alkenes. The C=C bond in alkenes cannot rotate. If there are different substituents at either end of the C=C bond then these can be arranged differently.

cis -1,2-dichloroethane trans -1,2-dichloroethane

‘the same side’ ‘on the other side’

The cis, trans designation is only applicable to simple alkenes. There are problems when more complex alkenes are considered, for example with the isomers shown below:

A better system assigns priorities to groups using a set of rules known as Cahn-Ingold-Prelog notation (CIP for short). The groups attached to the double bond of the alkene are prioritised according to their Atomic Number.

17

17

1

19 9

35 35

(Z)-1-bromo-2-chloro-1-fluoroethene (E)-1-bromo-2-chloro-1-fluoroethene

‘entgegen’, German for ‘opposite’‘zusammen’, German for ‘together’

31

5. Isomerism

If necessary, use the atomic numbers of the next atoms on the groups attached the double bond.

1

35

1

35

6

6

6

6

(Z)-1-bromo-2-methylbut-1-ene (E)-1-bromo-2-methylbut-1-ene

6

6

6

In this case, the next atoms in the right hand groups are both O.

Where there is a double bond to the next atom out (at the end of the double bond) is counted twice.

(Z)-2-(hydroxymethyl)-3-methylpent-2-enal

32

5. Isomerism

If there are identical atoms then list the atoms attached to these in atomic mass order and the re-establish the priority.

H H H

C H H

H H H

C H H

C H H

H H H

6 O H H

O O H

Stereoisomerism – Optical IsomerismIf a carbon atom in a molecule forms bonds to 4 different groups then Optical Isomers are possible. Carbon atoms that are bonded to 4 different groups are known as a Stereogenic Centres.

C

CH3

C

OH

ONH2

H

In the molecule above, the central carbon atom (*) is bonded to 4 different groups it is difficult to see how this can give rise to optical isomers since it is drawn flat. A 3D representation of the molecule however shows that there are two ways to arrange these groups in space.

*

33

5. Isomerism

These two molecules are ‘non-superimposable’ mirror images, which are Optical Isomers or Enantiomers.

Imaginary mirror

Optical isomers (enantiomers) often have very different activities in living systems.

R-(−)-Carvone S-(+)-Carvone

spicy aroma spearmint

* *

N

O

O

NH

H

O

O

(S)-Thalidomide

Teratogen (birth defects)

N

O

O

NH

H

O

O

(R)-Thalidomide Sedative

**

34

5. Isomerism

Stereogenic centres are easier to identify in displayed formulae - look for a carbon atom attached to 4 different groups. For example the alcohol below:

C C C C C

H

H

H

H

OH

H

H

H

H

H

H

H*

Stereogenic centres can be more difficult to spot in skeletal formulae.

1 23

45

*

HO

H H

CH3

H3C

H3C

CH3

CH3 H

How many stereogeneric centres are there in cholesterol?

35

5. Isomerism

Stereoisomerism – Drawing EnantiomersThe ‘Swap Method’.

1. Identify the stereogenic centre

2. Draw ‘wedges’ to show the groups in space.

3. Copy the diagram but swap the groups on the wedges.

The ‘Mirror Method’.

1. Identify the stereogenic centre

2. Draw ‘wedges’ to show the groups in space.

3. Copy the diagram as a mirror image.

mirror

mirror

mirror

36

5. Isomerism

Isomerism – A SummaryIsomers: molecules with the same molecular formula but different structures.

Structural Isomers: molecules with the same molecular formula but their atoms are connected differently.

Streoisomers: molecules with the same molecular and atoms connected in the same way but arranged differently in space.

Geometric Isomers: molecules that have different of groups around a carbon – carbon double bond

Optical Isomers (Enantiomers):molecules with stereogenic centres (connected to 4 different groups) that are nonsuperimposable mirror images.

Chain Isomers

Positional Isomers (Regio Isomers)

Functional Group Isomers

37

5. Isomerism

Reactions of Organic CompoundsThe reaction of organic molecules involve the breaking and making of bonds which involves the movement of paired or unpaired electrons between atoms. Sometimes bonds are broken and made simultaneously, in other cases bonds may be broken, and then new ones formed. These processes are captured in organic reaction mechanisms (also known as ‘arrow pushing’ mechanisms).

Breaking Bonds – Heterolytic Fission (Heterolytic Cleavage)A bond is a pair of electrons which is ‘shared’ between two nuclei.

A B

One way that a bond can break is for both electrons to goes to one of the atoms of the bond. This is more likely to occur when the atoms is a bond have significantly different electronegativities.

A B A B+ -

+

Curved arrows are used to show the movement of electrons in reactions. They should clearly show where the electrons come from and where they end up. In the case of heterolytic fission the movement of an electron pair is shown using a double headed ‘curly arrow’

A B A B+ -

+

Breaking Bonds – Homolytic Fission (Homolytic Cleavage)When two bonded atoms have electronegativities that are close then the bond can break so that each atom has an unpaired electron, each becoming a ‘free radical’.

A B A B+

A single-headed arrow (‘fish hook’) shows how the unpaired electrons move.

A B A B+38

6. Reactions of organic compounds

Nucleophiles in Reactions

A nucleophile (nucleus loving) is a species that has an electron pair that it can donate to form a covalent bond. Nucleophiles have high electron density and can be either neutral or negatively charged. Nucleophiles ‘attack’ atoms that have a whole, or partial positive charge. Some examples are shown below:

Cl-, OH-, NH3, CN-

In reaction mechanisms it is important to show the electron pair on the nucleophile along with a curly arrow being used to start at the electron rich lone pair and end up on an atom where the bond is formed

HOC X

+ -

Electrophiles in ReactionsAn electrophile (electron loving) is a species with low electron density that can accept an electron pair to form a covalent bond. Electrophiles can be positively charged or neutral. Some examples are shown below.

HBr, NO , SO2 3

+

Electrophiles will interact with electron-rich groups, such as carbon-carbon double bonds.

+ -

C C

H Br - +

39


Free Radicals in ReactionsFree radicals are highly reactive (short lived) species that have an unpaired electron. The unpaired electron is shown as a single dot. Examples of free radicals are:

Cl, Br, CH3

Free radicals can be produced by the use of UV or heat and catalysis. This is called initiation.

Cl Cl 2Cl

Free radicals can be interact with molecules to generate different free radicals. This called propagation, which is part of a chain reaction.

Cl H CH3 HCl + CH3

Free radicals can also react with each other. Because this destroys two radicals it is called termination.

CH3Cl ClCH3

40


AlkanesThe alkanes are a series of saturated (contain only single bonds) hydrocarbons (contain only carbon and hydrogen atoms). The general formula for an alkane is CnH2n+2 (cycloalkanes CnH2n).

Alkanes can be either straight- or branched chains. The straight chained alkanes form what is known as a homologous series.

CH4

CH3CH

3

CH3CH

2CH

3

CH3CH2CH2CH3

CH CH CH CH3 2 2 2CH3

methane

ethane

propane

butane

pentane

etc.

https://i.stack.imgur.com/hmmYd.png

The most important source of alkanes is crude oil, a mixture of alkanes and other hydrocarbons. These are separated into ‘fractions’ using a fractional distillation process. Each fraction contains hydrocarbons with a range of chain lengths. These can then be separated further into pure alkanes.

41

7. Alkanes

Physical PropertiesThe alkanes have relatively low boiling points since they are non-polar (C and H have similar electronegativities) and the intermolecular forces are weak Van der Walls’ forces. The boiling points increase with chain length.

-200

-300

-100

0 5 10 15 20 25 45 50 55 60 65

700

600

500

400

300

200

100

0

30 35 40

Boiling Point of Alkanes

BP ( C)o

Chain Length

Heptane BP = 98 Co

Decane BP = 174 Co

As the carbon chain gets longer there are greater Van der Waals’ forces between neighbouring molecules. The increase in energy needed to break these intermolecular bonds mean that the boiling points increase.

There are similar trends with melting points, viscosity (runniness) and density.

42

7. Alkanes

Branched ChainsAlkanes can be branched, that is to have carbon chains attached to other carbon chains. Below are the three isomers of the hydrocarbon C H which show increased branching

5 12

BP = 36.1C BP = 27.9C BP = 9.5C

Branching means that the molecules are less able to pack closely together and consequently the intermolecular forces (Van der Waals) are lower for a particular molecular mass.

ReactionsOne of the characteristics of alkanes is that they are relatively unreactive. This is because the electronegativities of C and H are similar so that electrons are distributed between them fairly evenly.

C H2.12.5

Because C-C and C-H bonds are non-polar (evenly charged) there are no regions of high or low electron density that can be attacked by electrophiles or nucleophiles. Alkanes however can react with free radicals.

43

7. Alkanes

Free Radical SubstitutionAlkanes react with chlorine and bromine at room-temperature in the presence of UV light (sunlight is sufficient) to produce HCl and a mixture of chloroalkanes.

For example: CH4 + Cl2 CH3Cl + HCl

CH3Cl + Cl2 CH2Cl2 + HCl

CH2Cl2 + Cl2 CHCl3 + HCl

CHCl3 + Cl2 CCl4 + HCl

h

h

h

h

The degree of substitution depends on conditions. With an excess of alkane the predominant product is the monosubstituted haloalkane.

Excess of ChlorineExcess of Alkane

More multiple substitutionsLess multiple substitutions

Free radical substitution reactions are chain reactions involving ‘initiation’ (where free radicals are produced), ‘propagation’ (where free radicals react with molecules to make different radicals) and ‘termination’ (where free radicals react with each other).

Initiation

The relatively weak bonds in Cl and Br (compared to C-C and C-H) mean that these can be broken easily so the homolytic fission of these is the first step (initiation).

2 2

Cl-Cl 242 kJ/molBr-Br 193 kJ/molC-C 346 kJ/molC-H 413 kJ/mol

Cl2 2Clh

44

7. Alkanes

e.g.

Propagation

The halogen radicals then collide with alkane molecules and extract a hydrogen atom to form HCl and an alkyl radical.

Cl + CH4 HCl + CH3

The alkyl radicals can then react with Cl2 molecules.

Cl2 + CH3 CH3Cl + Cl

Propagation reactions always consume one radical and make a new one (a chain reaction).

Termination

When two radicals combine the form a molecule so this ends the chain reaction. There are several possible termination reactions.

Cl + Cl Cl2

Cl + CH3 CH3Cl

CH3 + CH3 CH3CH3

45

7. Alkanes

Overall Reaction (Mono-substitution)

Cl2 2Clh

Cl + CH4 HCl + CH3

Cl2 + CH3 CH3Cl + Cl

Cl + Cl Cl2

Cl + CH3 CH3Cl

CH3 + CH3 CH3CH3

Initiation

Propagation

Termination

Free Radical Substitution – More Complex AlkanesEthane

All six C-H bonds are identical, therefore there is a single mono-substituted product (chloroethane).

C

H

H

C

H

H

H H + Cl2h

C

H

H

C

H

H

H Cl

If a second substitution takes place there are two possible products.

C

H

H

C

H

H

H Cl + Cl2h

C

Cl

H

C

H

H

H Cl C

H

H

C

Cl

H

H Cl+

46

7. Alkanes

With larger alkanes, substitution is even more complicated.

Propane

Mono-substation gives two products; 1-chloropropane and 2-chloropropane.

H

C

H

H

C

H

H

C

H

H Hh

+ Cl2

H

C

H

H

C

H

H

C

H

H Cl

H

C

H

H

C

Cl

H

C

H

H H +

55% 45%

Competing Factors1. Probability – favours 1-chloropropane (6 primary H atoms, 2

secondary atoms).2. Stability of intermediate radicals (most stable more likely to form).

H

C

H

H

C

H

C

H

H H

H

C

H

H

C

H

H

C

H

H

Secondary radicals most stable – favours 2-chloropropane.

Relative stability of radicals

If there is a possibility of several radicals being formed in a propagation step, then the one formed preferentially has an effect on the distribution of the products.

Stability of Radicals

Tertiary Secondary Primary> >

C-H Bond Strength

~420 kJ mol-1~400 kJ mol-1~390 kJ mol-1

47

7. Alkanes

2 Methyl propane

H C

H

H

CH

H

C

H H H

CH

H h+ Cl2 H C

H

Cl

CH

H

C

H H H

CH

H

H C

H

H

CH

Cl

C

H H H

CH

H+

68%32%

Based on probability alone, would expect 9:1 ratio of primary haloalkane to tertiary haloalkane.

Tertiary haloalkane produced via the most stable (secondary) alkyl radical.

H C

H

CH

H

C

H H H

CH

H

‘Selectivity’ – Chlorination vs Bromination

H

C

H

H

C

H

H

C

H

H Hh

+ Cl2

H

C

H

H

C

H

H

C

H

H Cl

H

C

H

H

C

Cl

H

C

H

H H +

55% 45%

H

C

H

H

C

H

H

C

H

H Hh

+ Br2

H

C

H

H

C

H

H

C

H

H Br

H

C

H

H

C

Br

H

C

H

H H +

97% 3%

Free radical bromination is much more selective than chlorination, so bromination is useful in synthesis, as it reduces the unwanted side products.

48

7. Alkanes

Bromination of Methylcyclohxane

h+ Br2

49

7. Alkanes

Cracking‘Cracking’ is a reaction that is used to break down long chain alkanes (obtained by fractional distillation of crude oil) into smaller molecules that can be used as fuel or useful materials such as alkenes (see later) which can be used in synthesis. Cracking is carried out at high temperatures and pressures (thermal cracking) or in the presence of a catalyst (catalytic cracking). Cracking involves the homolytic fission of a C-C bond (since it is weaker than C-H) followed by chain reactions involving the creation of further radicals. The example below shows the cracking of octane.

+

CH3

CH2

CH2

CH2

CH2

CH2

CH2

CH3

CH3

CH2

CH2

CH2

CH2

CH3CH2 CH2

CombustionAlkanes burn with a considerable evolution of heat.

CH + 2O CO + 2H O4 2 2 2 DH = -882 kJ/mol

All alkanes will burn, but as the chain length increases the larger Van der Waals forces means that they vaporise less easily and so are more difficult to ignite.

CH + 1.5 O CO + 2H O4 2 2

CH + O C + 2H O4 2 2

The equation above shows ‘complete combustion’ which means that there is enough oxygen present in the reaction to oxidise both C and H fully. When oxygen is restricted the carbon atoms in alkanes will not be fully oxidised so carbon monoxide, or even carbon is formed. This is ‘incomplete combustion’.

50

7. Alkanes

AlkenesAlkenes are a class of hydrocarbons that contain a C=C bond. These are ‘unsaturated’ as they have not got the maximum possible amount of hydrogen atoms attached to the carbon chain. The general formula for an alkene is C H with n>1 (cycloalkenes CnH2n-2). Apart from the simplest two alkenes (n= 2, ethene and n=3 propene) the double bond can occupy different position in a chain (structural isomers).

n 2n

n=2

n=3

n=4

n=5

n=6

ethene

propene

but-1-ene but-2-ene

pent-1-ene pent-2-ene

hex-1-ene hex-2-ene hex-3-ene

*

*

* *

* cis/trans (E/Z) isomers also possible

ReactivityAlkenes are more reactive than alkanes because the energy required to break the double bond is less than twice the energy needed to break a single bond.

H

C

H H

H

C𝜎

𝜋

C-C 346 kJ/molC=C 612 kJ/mol

The bond in alkenes is a region of high electron density and so is therefore prone to attack from electrophiles and oxidising agents.

51

8. Alkenes

Addition ReactionsThe characteristic type of reactions alkenes undergo are ‘addition reactions’ which involve the double bond. These often involve electrophiles.

Hydrogenation reactions add two hydrogens and so an alkane is produced. These reactions require a catalyst such Ni, Pt or Pd.

H H

H HNi

H2+Ni

Hydrogenation

Addition of HCl, HBr and HIAlkenes react readily (room temperature) with concentrated aqueous hydrogen halides to form haloalkanes.

H

H

= Cl, Br or I

For example

HC

H

H

H+ HBrC H C

H

H

Br

C

H

H

(aq)

1-bromoethane

52

8. Alkenes

The reaction between alkenes and hydrogen halides is an example of ‘electrophilic addition’. The hydrogen halide is polar making the hydrogen atom electrophilic.

H Br + -

HC

H

H

HC

H C

H

H

H

C

H+

Br -

H C

H

H

H

C

H+

Br -

H C

H

H

Br

C

H

H

The hydrogen in HBr bears a δ+ charge, and is electrophilic. Two electrons are transferred from the π bond to form a new C-H bond. The H-Br bond breaks at the same time. This results in the formation of a carbocation. This is the slowest part of the reaction (the rate determining step) because it has the highest activation energy (Ea ).

The carbocation is thenattacked by a Br- ion (a nucleophile) to form a C-Br bond.

53

8. Alkenes

The reaction between HBr and ethene forms only one product. With asymmetrical alkenes such as propene there are two possibilities.

H

CH

H

H+ HBrC

H C

H

H

Br

C

H

H

(aq)

1-bromopropaneC

C

H

H

H C

H

H

Br

C

H

H

2-bromopropane

C

H

H

H

H

In this case the predominant product is 2-bromopropane, with only a small amount of 1-bromopropane in the product mixture.

The predominant product of the reaction between hydrogen halides (HCl, HBr and HI) can be predicted using Markovnikov’s Rule:

When HX adds across an asymmetric double bond, the major product is the molecule in which hydrogen adds to the carbon atom in the double bond with the greater number of hydrogen atoms already attached to it.

H

CH

H

H+ HBrC

(aq)

CH

H

H C

H

H

Br

C

H

H

C

H

H

1 2

54

8. Alkenes

Markovnikov’s rule can be explained by considering the stability of the of the cabocations produced in each possible route.

H

CH

H

HC

CH

H

HBr(aq)

H C

H

H

Br

C

H

H

C

H

H

H C

H

HC

H

H

C

H

H

Br-

-

+

+H C

H

H

Br

C

H

H

C

H

H

H C

H

H

Br

C

H

H

C

H

H

A

B

primary carbocation

secondary carbocation

The secondary carbocation is more stable than the primary carbocation due to having more carbons attached to the positive carbon atom, carbon atoms in alkyl groups being electron releasing. Because this is the most stable intermediate, route B, leading to 2-bromopropane is favoured.

CH

CCH3

CH3

CH3

1-bromopropane

2-bromopropane

2-methylbut-2-ene

HBr(aq)

C H

Br

C

H

CH3

CH3CH3

+

2-bromo-2-methylbutane

Markovnikov’s rule predicts that the reaction between 2-methylbut-2-ene and HBr should produce predominantly 2-bromo-2-methylbutane, which it does.

This again can be explained by the stability of the carbocation leading to the predicted product being more stable than the alternative.

C HC

H

CH3

CH3CH3

C HC

H

CH3

CH3CH3

+ +

secondary carbocation

tertiary carbocation

This demonstrates the order of stability of carbocations:

tertiary carbocation

secondary carbocation primary carbocation> >

3[R C]

+[R CH]

2+

[RCH ]2+

55

8. Alkenes

Addition of Halogen (Br or I )2 2

X = Cl or BrX X

X X

For example

HC

H

H

H+ BrC H C

H

H

Br

C

H

2

1,2-dibromoethane

Br

ethene

Alkenes react readily with chlorine or bromine at room temperature to yield dichloro- or dibromo-alkanes.

This is a further example of electrophilic addition. Although the Cl-Cl or Br-Br bond is not polar, a dipole is ‘induced’ as it approaches the electron rich 𝜋 bond.

HC

H

H

HC

Cl

Cl

+

-

H CH

HH

C

Cl

+

Cl-

H C

H

H

Cl

C

H

Cl

In the case of addition of bromine, there is evidence for an intermediate called a bromonium ion.

HC

H

H

HC

Br

Br

+

-

H CH

HH

C

Br

+

Br-

H C

H

H

H

C

Br

Br

H CH

HH

C

Br+

bromonium ion

56

8. Alkenes

H Br

Br H

H H

Br

The evidence for the presence of the bromonium ion in the reaction between bromine and alkenes comes from a consideration of the product of the reaction with cyclic alkenes (e.g. cyclopentene).

HHBr

Br

+

-

H

HBr

+

Br

H

HBr

+-

Br

Br

H

BrBr

H Br

Br H

H

The large bromine atom prevents attack from the lower side so only one product is possible.

If the bromonium ion was not involved then two products would be seen.

The reaction however only yields one product which supports the involvement the bromonium ion.

Addition of H O2

Alkenes react with steam under high pressures (6MPa) and high temperatures (600K) using H PO as a catalyst on an inert support to produce alcohols.

3 4

HC

H

H

H+ H OC H C

H

H

OH

C

H

H

ethene

2

H+

ethanol

CH

H

H+ H OC H C

H

OH

C

H

H

propene

2

H+

Propan-2-ol

CH3C

H

H

H

*

* The predominant product is arrived at via the most stable (in this case secondary) carbocation.

For example

57

8. Alkenes

+H

+

OH2

O

HH +OH

+H

The catalyst phosphoric acid supplies a proton (H ) which acts as an electrophile in the first step. The carbocation is then attacked by water (acting as a nucleophile). The final step is the loss of a proton, which means that the acid’s role has been catalytic.

Addition of H O and X (X= Cl or Br)

+

2 2

If chorine or bromine, along with water are allowed to react with an alkene then a halohydrin is formed.

X , H O

= Cl or BrX

OH X

2 2

For example

Cl , H O2 2

OH

Cl

propene 1-chloropropane-2-ol

+ HX

+ HCl

58

8. Alkenes

Cl Cl Cl

Cl

+

-OH2

Cl

O

HH +

Cl

OH

+H

The first step of the reaction is similar to the addition of halogens to alkenes. The chlorine (induced dipole) acts as an electrophile which attacks the double bond. The carbocation formed (the most stable), instead of being attacked by the halide, is attacked by water. The final step is the release of a proton.

Addition Polymerisation Under suitable conditions shorter alkenes (monomers) can react with each other to form long chains (polymers). The structure (and therefore properties) are dependent upon the monomer and the conditions. Catalysts for polymerisation include TiCl and Al(C H ) .

4 2 5 3

C C

HH

H H

C C

H H

H H

n

n

Ethene Poly(ethene) - Polythene

C C

ClH

C C

H Cl

H H

n

n

H H

Chloroethene - vinyl chloridePoly(chloroethene – PVC – Poly(vinylchloride)

n=several thousand

C C

CH3H

H H

H CH3

C C

H H

n

n

Propene Poly(propene)

Some examples

59

8. Alkenes

HaloalkanesHaloalkanes are compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms (F, Cl, Br or F). These have many uses (e.g. solvents, fire retardants) but also useful as intermediates in the synthesis of other compounds (e.g. polymers and pharmaceuticals).

H C

H

H

Cl

C

H

H

C

H

H

H C

H

H

Cl

C

H

H

C

H

H

H C

H

H

Cl

C

H

H

C

H

H

C

HH

Primary (10)haloalkanes have the halogen at the end of the chain.

Secondary (20)haloalkanes have the halogen within a chain.

Tertiary (30)haloalkanes have the halogen at a branching point within the chain.

Cl

ClCl

SynthesisFrom Alkenes (addition of conc. aq. HCl, HBr or HI)

HC

H

H

H+ HClC H C

H

H

ClC

H

H

(aq)

From Alcohols (reaction with NaBr and conc. H SO )2 4

H C

H

OHC

H

H

C

H

H H

+ HBr H C

H

BrC

H

H

C

H

H H

H O2+

From Alcohols (reaction with PCl , PI , or SOCl )5 3 2

3 CH3CH2OH + PI3 → 3 CH3CH2I + H3PO3

CH3CH2OH + PCl5 → CH3CH2Cl + POCl3 + HCl CH3CH2OH + SOCl2 → CH3CH2Cl + SO2 +HCl

60

9. Haloalkanes

Nucleophilic Substitution Reactions

Bond Polarity

The electronegativity of the halogens are in the order of F > Cl > Br > I which makes the most polar (most charge separation) carbon-halogen bond C-F.

Bond Strength

C-F 467 kJ/mol

C-Cl 340 kJ/mol

C-Br 280 kJ/mol

C-I 240 kJ/mol

Haloalkanes can undergo ‘substitution’ reactions with nucleophiles such as HO , NH , and CN. These reactions exchange the halogen for a different functional group and so are useful in organic synthesis.

- -3

H C

H

H

XC

H

H

C

H

H

OH-

+ H C

H

H

OHC

H

H

C

H

H

Cl-

+

For example

To understand the relative reactivity of the haloalkanes it is important to consider the Carbon-Halogen bond

Although the C-F is the most polar C-X bond, its high bond strength makes it very un-reactive towards nucleophiles. The order of reactivity instead follows the reverse order of C-X bond strength with iodoalkanesbeing the most reactive.

RI > RBr > RCl >> RF

61

9. Haloalkanes

Nucleophilic Substitution Reactions -Mechanism

There are two possible mechanisms for the nucleophilic substitution of haloalkanes:

BrC

HO-

+ OHC Br-

+

This involves the donation of a lone pair from the nucleophile to the partially positively charged C atom to form a new bond with the simultaneous breaking of the C-halogen bond.

This reaction is a single step process that involves both the haloalkane and the nucleophile (bimolecular) in the slowest (rate-determining step). The rate of reaction depends on the concentration of both the haloalkane and nucleophile (second order).

S 2 MechanismN

Rate a [RX][Nuc]

Ener

gy

Reaction

Ea

62

9. Haloalkanes

Rate = k[RX][Nuc]

HO-

BrC

Me

Et

H

BrC

Me

EtH

HO - -

BrC

Me

EtH

HO-

In the S 2 mechanism the nucleophile approaches from the rear of the C-Halogen bond. As the new bond is between the nucleophile and the carbon is being formed, the bond between the carbon and the halogen is being broken. The result is that the stereochemistry is inverted.

N

The S 2 mechanism is followed by the reaction between nucleophiles and primary, and some secondary haloalkanes.

N

An alternative mechanism involves the heterolytic fission of Carbon-Halogen bond to yield a carbocation intermediate. The intermediate is then attacked by the nucleophile.

S 1 MechanismN

BrC OH OHC+C2

- H+

- Br-

63

9. Haloalkanes

This mechanism is a two step process with the formation of the carbocation (unimolecular) being the slowest (rate determining) step. The rate of reaction depends only on the concentration of the haloalkane (first order).

Rate a [RX]

Ener

gy

Reaction

Ea

The carbocation intermediate formed by the loss of the halide has a trigonal planar structure which can be attacked by the nucleophile from either side of the plane. This means that if the haloalkane is a stereoisomer, a mixture stereoisomers are produced.

Cl

CCH3

C H2 5

C H3 7

C CH

C H3

C H2 5

3 7

OH2

OH2

A

B

OH

C

CH32C H

5

C H3 7

OH

C

CH32C H

5

C H3 7

-H+

-H+

-Cl-

64

9. Haloalkanes

Rate = k[RX]

S 1 or S 2 Mechanism?N N

There is a competition between the two different nucleophilic mechanisms (along with elimination reactions – see later) and the outcome is determined by several factors.

a) Steric EffectsThe S 2 mechanism requires a nucleophile to approach the carbon atom from the rear of the bond with the halogen. This is easier if the haloalkane is primary or secondary.

HO-

BrC

Me

H

HHO

-BrC

Me

H

EtHO

-BrC

Me

Et

Et

least hindered most hindered

Primary SecondaryTertiary haloalkanes do not undergo the S 2 mechanism

>

N

b) Stability of the CarbocationThe S 1 mechanism involves the heterolytic fission of the carbon-halogen bond to yield a carbocation intermediate. Alkyl groups are able to donate electron density to the positively charged carbon atom and so have a stabilising effect on the intermediate. The more stable the intermediate – the more likely the S 1 route.

N

S 2 Favoured in order of:N

C CH 3

+H

H

C CH 3

+H

CH3

C CH 3

+CH

CH3

3

N

most stable least stable

PrimarySecondaryTertiary > >S 1 Favoured in order of:N

N

65

9. Haloalkanes

Examples of Nucleophilic SubstitutionReaction with AmmoniaThe lone pair on the nitrogen atom of ammonia means that it can act as a nucleophile. An excess of ammonia in ethanol yields a primary amine.

Br+ NH3

NH2 + HBr

Reaction with CyanideThe reaction between haloakanes and KCN in ethanol yields a nitrile, an important synthetic intermediate that increases a carbon chain by 1 carbon atom.

Br

+ KCN + KBrC

N

Reaction with AlkoxidesAlkoxides (RO-, e.g. ethoxide; CH3CH2O-) can act as a nucleophile and these react with alkyl halides to produce ethers. Because of the possibility of side reactions (eliminations – see later) the reaction is used with primary alkyl halides only.

++

Reaction with ThioalkoxidesThioalkoxides (RS-, e.g. thiomethoxide; CH3S-) react in a similar way to alkoxides to make thioethers.

++

66

9. Haloalkanes

Elimination ReactionsHaloakanes are able to eliminate a molecule of hydrogen halide and in doing so form an alkenes.

H C

H

H

Br

C

H

H

C

H

H

+ HO-

H

C

H

H

BrC

H

H

C

H

H O+ -2

Elimination reactions and nucleophilic substitution make take place together with the reaction conditions having a great effect on which type of reaction predominates (see later).

There are two possible mechanisms for the elimination of HX from haloalkanes.

E1 MechanismAs is the case with S 1 reactions, haloalkanes can undergo heterolytic fission to produce a carbocation intermediate.

If a strong base is present in the reaction mixture then a proton can be taken from the carbon next to the positively charged carbon atom to form an alkene.

H C

H

H

Br

C

H

H

C

H

H

H C

H

H BrC

H

H

C

H

H+

+-

HO-

H O+

H

C

H

H

C

H

H

C

H2

H C

H

HC

H

H

C

H

H+

+

N

67

9. Haloalkanes

The slowest (rate determining) step in the reaction is the formation of the carbocation so the rate of reaction is only dependent on the concentration of the haloalkane.

Rate a [RX]

In some alkenes there are several hydrogen atoms that could be lost from the carbocation, each yielding a different product.

H C

H

H

Br

C

H

H

C

H

H

C

H

H

H C

H

HC

H

C

H

H

C

H

H+

+ Br-

A B

2-bromo -2-methylbutane

2-methylbut-2-ene

CH

H

HCH

H

H C

H

HC

H

C

H

H

C

H

H+

HO-

CH

H

H

H C

H

HC

H

C

H

H

C

H

H+

HO-

CH

H

H

H

C

H

H

C

CH

HC

H

H

C

HH

H

H

C

H

C

H

C

H

HC

H

H

C

H H

H

2-methylbut-1-ene

major product

minor product

The favoured product is the one with the most highly substituted alkene.

68

9. Haloalkanes

Rate = k[RX]

E2 MechanismIn the E2 mechanism the carbon-carbon double bond is broken at the same time as the formation of the carbon-carbon double bond.

H C

H Br

C

H

HO-

H

H

Br

H

C C

H

H

H

H O+ +-

2

Since both the alkene and the base are involved in the rate determining step the reaction is second order.

Rate a [RX][Base]

Haloalkanes – Competition Between Substitution and EliminationBecause the reagents used for substitution and elimination are similar’ both reactions will happen to some extent. The predominant route depends on various factors.

The HaloalkanePrimary – Mostly substitutionSecondary – Both substation and eliminationTertiary – Mostly elimination

Br HO-

OH +

Br

HO-

OH

+

Br

HO-

major product

mixture

only product

69

9. Haloalkanes

Rate = k[RX][Base]

SolventsBoth nucleophilic substitution and elimination reactions need polar solvents to dissolve the reactants used. There are two classes of these and these are important in determining the course of a reaction.

Polar-protic solvents Polar-aprotic solvents

Have O-H, or NH groups so can undergo H-bonding.

e.g. H2O, CH3CH2OH, NH3.

Do not have O-H, or NH groups so unable undergo H-bonding.

e.g. CH3COCH3 (propanone), CH3CN (Acetonitrile), (CH3)2NCHO (Dimethylformamde or DMF)

70

9. Haloalkanes

2. If Substitution – SN1 or SN2?

tertiary haloalkanes > secondary haloalkanes > primary haloalkanes

SN2One step mechanism. Rate a [RX][Nuc]Inversion of stereochemistry.

SN1Two step mechanism involving carbocation intermediate.Rate a [RX]Produces mixture of enantiomers.

Polar-protic solventse.g. ethanol

polar aprotic solventse.g. propanone, CH3CN,

dimethyl formamide (DMF)

SN1, SN2, E1 or E2 – How will you know?

Because the reagents used for substitution and elimination are similar both reactions will happen to some extent. The predominant route depends on various factors:

1. Substitution vs Elimination

Substitution Elimination

primary haloalkanes > secondary haloalkanes >> tertiary haloalkanes

polar aprotic solventse.g. propanone, CH3CN

polar protic solventse.g. ethanol

lower temperatures higher temperatures

71

9. Haloalkanes

3. If Elimination – E1 or E2?

E1Two step mechanism involving carbocation intermediate.Rate a [RX]

E2One step mechanism.Rate a [RX][Base]

tertiary haloalkanes > secondary haloalkanes > primary haloalkanes

72

9. Haloalkanes

Alcohols are a class of organic compound that contain an –OH group attached to a saturated carbon atom with only other C and H atoms attached. Alcohols can be primary, secondary of tertiary.

The –OH group is commonly found in organic molecules but these are not necessarily alcohols. None of the below are alcohols.

primary secondary tertiary

OHOH

OH

O

Alcohols

Physical PropertiesAlcohols have much higher boiling points than alkanes of the same chain length.

CH3CH3 CH3CH2OHBP = - 89°C BP = 78°C

This can be explained by the presence of the polar O-H bond which allows hydrogen bonding between alcohol molecules. This is a much stronger type of intermolecular bond than the Van der Waal’s forces that are only present between alkane molecules.

H

H

OC

H

C

H

HH

H

H

O C

H

C

H

HH

+

-

+

-73

10. Alcohols

Shorter chain alcohols (up to n<4) are water soluble (miscible) also due to hydrogen bonding with water. For example both methanol and water are miscible with water in all proportions.

H

H

OC

H

C

H

HH +

-

O

H

H

+

-

+

Longer alkyl chains interfere with this bonding reducing their solubility.

SynthesisEthanol can be prepared by the fermentation of carbohydrates. This is carried out under anaerobic conditions using enzymes present in yeast and moderate temperatures (ca. 35°C). The fermentation takes several days and yield solutions up to about 10% ethanol.

C6H12O6 2C2H5OH 2CO2+

Other alcohols (including ethanol) can be made by the reaction of alkenes with steam at high temperatures (600 °C) and high pressures (60 atmospheres) using silica coated with phosphoric acid (H3PO4) as a catalyst.

OH

OH

ethene

propene

ethanol

Propan-2-ol

H2O / H +

H2O / H +

Note: The addition of water to alkenes the H always attaches to the carbon with the most H atoms already on it. This means that some alcohols cannot be made by this process.

74

10. Alcohols

Reaction with Oxygen (Combustion)All alcohols will undergo complete combustion when they are burned in a plentiful supply of air. These combustion reactions are very exothermic and so alcohols are used as fuels.

C2H5OH 3O2 2CO2 3H2O+ + ΔH = -1367 kJ/mol

OxidationPrimary and secondary alcohols can be oxidised with acidified (sulfuric acid) potassium dichromate(VI) (K2Cr2O7) and heating.

Acidified potassium dichromate(VI) is orange and when it has reacted there is a colour change to green. This is the basis of a test to distinguish between primary/secondary alcohols (colour change) and tertiary alcohols (no colour change).

Primary AlcoholsPrimary alcohols are oxidised firstly to aldehydes, and then to carboxylic acids.

OHO

H

O

OH

[O] [O]

Secondary AlcoholsSecondary alcohols are oxidised to ketones.

OH O

[O]

Tertiary AlcoholsAre not oxidised by K2Cr2O7/H2SO4.

75

10. Alcohols

NOTE: The use of the oxidising pyridinium chlorochromate (PCC) will oxidise primary alcohols to aldehydes and no further.

Reaction with HBr or HClPrimary alcohols react with concentrated HCl or HBr to yield chloro- or bromo-alkanes. If HBr is needed it is prepared in situ by the reaction of sodium bromide and sulfuric acid.

NaBr + H2SO4 → NaHSO4 + HBr

The HBr then reacts with the alcohol by replacing –OH with –Br. This is used mainly to prepare tertiary bromides from tertiary alcohols.

OH NaBr/H2SO4Br

OH HCl Cl

Reaction with Phosphorus HalidesAn easier way to make haloalkanes from alcohols is to use a phosphorus halide (PCl5, PCl3, PBr3, PI3) at room temperature.

3 CH3CH2OH + PCl3 → 3 CH3CH2Cl + H3PO3

CH3CH2OH + PCl5 → CH3CH2Cl + POCl3 + HCl 3 CH3CH2OH + PBr3 → 3 CH3CH2Br + H3PO3

3 CH3CH2OH + PI3 → 3 CH3CH2I + H3PO3

Reaction with Thionyl ChlorideA similar reaction to those above occurs when thionyl chloride (SOCl2) reacts with alcohols at room temperature.

CH3CH2OH + SOCl2 → CH3CH2Cl + HCl + SO2

76

10. Alcohols

DehydrationThe removal of a water molecule from an alcohol will yield an alkene.

H H

O

C

H

C

H

H HH H

H OCH

CH

+2

Dehydration using Aluminium Oxide as a CatalystThe simplest way to dehydrate an alcohol is to pass the alcohol vapour over aluminium oxide (Al O ) at 400°C.2 3

mineral wool soaked in ethanol

aluminium oxide

hard glass boiling tube

etheneBunsen valve

Diagram adapted from: LearnChemistry (RSC)

Dehydration using an Acid CatalystAlcohols can be dehydrated by heating them with a strong acid such as sulfuric or phosphoric acid.

H H

O

C

H

C

H

H HH H

C

H

C

H

conc. H SO2 4

OHconc. H PO

3 4

Some alcohols will yield a number of products, for example butan-2-ol.

OH

conc. H PO3 4 + +

butan-2-ol but-1-ene (E)-but-2-ene (Z)-but-2-ene77

10. Alcohols

EsterificationEsters (RCOOR’) are derivatives of carboxylic acids (RCOOH) where the H is replaced by a hydrocarbon group (for example an alkyl group).

Alcohols with Carboxylic AcidsEsters can be produced by heating a carboxylic acid and an alcohol in the presence of a strong acid as a catalyst, however these reactions are slowand reversible. The reaction eliminates a smaller molecule (water) and is an example of a condensation reaction.

C C

O

O H

H

H

H

conc. H SO2 4

H HO C

H

C

H

H H

+C C

O

O H

H

H

H

HC

H

C

H

H H+ H O2

Alcohols with Acyl Chlorides (Acid Chlorides)Alcohols react vigorously with acyl chlorides (RCOCl) at room temperature to form an ester and fumes of HCl. The advantage of this reaction is that it is fast and goes to completion.

Alcohols with Acid AnhydridesAcid anhydrides react with alcohols than acid chlorides so need to be heated. The advantage is that they release ethanoic acid rather than the more corrosive HCl.

O

Cl

OH

O

OHCl++

ethanoyl chloride ethanol ethyl ethanoate

O

O

O

OH

O

O+

ethanolethanoic anhydride ethyl ethanoate

+

O

OH

78

10. Alcohols

Arenes and Aromatic ChemistryArenes are a class of cyclic hydrocarbons with one example being benzene (C H ).6 6

The term ‘aromatic’ is an old reference to their strong sweet smells but now refers to a special kind of bonding that involves delocalised electrons in certain cyclic compounds

CH3 CH3

CH3

Some other arenes.

methylbenzene 1,2-dimethylbenzene

The structure of Benzene

Michael Faraday

August Kekulé

Benzene, a natural constituent of crude oil, was first isolated by Michael Faraday in 1825. Its formula was established to be C H but its structure was elusive to chemists.

6 6

Various attempts at the structure of benzene

August Kekulé (1865) eventually arrived at a cyclical structure that had alternate single and double bonds between the carbon atoms.

79

11. Arenes

Although the Kekulé structure had appropriate bonding to satisfy the known formula of benzene there were some major issues. For example, if benzene contained three carbon-carbon double bonds then they should undergo addition reactions (like alkenes).

Benzene however undergoes substitution reactions.

H

H

H

H

+ Br2 Br

Br

BrBr2

FeBr3 (cat.)

A further problem was found with 1,2-dichlorobenzene which would have two isomers, where only one is observed.

Cl

Cl

Cl

Cl

In response to this Kekulé proposed that the two theoretical isomers would be in rapid equilibrium.

Cl

Cl

Cl

Cl

80

11. Arenes

X-ray diffraction studies carried out in the mid 1900’s showed that all the carbon-carbon bonds were the same length (0.139 nm) which lies in-between the typical value of a single bond (0.154 nm) and a double bond (0.134 nm). Furthermore the benzene molecule was found to be planar with all the C-C-C bond angles being 120°.

81

11. Arenes

The hydrogenation of benzene provided further evidence for the Kekulé structure being incorrect.

H2+

cyclohexene cyclohexane

ΔH = -118 kJ/mol

From this it can be predicted that the enthalpy change for the hydrogenation of Kekulé benzene (with it’s 3 double bonds) would be three times this value.

+ 3H2

cyclohexaneKekulé benzene

ΔH = -354 kJ/mol

The experimental value for the enthalpy change however is considerably less (-205 kJ/mol) than the theoretical value.

enthalpy

Kekulé benzene

cyclohexane

benzeneTheoretical ΔH = -354 kJ/mol

Experimental ΔH = -205 kJ/mol

This means that benzene is more stable than the Kekulé model suggests.

82

11. Arenes

The experimental evidence led Linus Pauling (1931) to suggest that benzene was in fact half way between the two possible Kekuléstructures as a resonance hybrid (not in equilibrium). This is shown by the diagram below on the right.

Benzene – The Resonance Model

The structure of benzene is best described as a delocalised systemwhere the electrons are shared around the ring. This imparts stability and helps to explain its lack of reactivity compared to alkenes.

=

H

HH

H

H H

C

C

CC

C

C

Sp hybridised C atom

H atom 1s orbital

2

The model has each of the carbons of the ring with sp hybridisation. These overlap with other carbon sp orbitals and the 1s orbitals of six H atoms to form a planar structure held together by 𝜎 bonds.

2

2

After the 𝜎 bonds have formed, each C atom has an electron in a 2p orbital which overlap to produce a delocalised 𝜋 system above and below the plane of the ring.

𝜋

2p𝜎 bonds

H H

H H

H H

H H

83

11. Arenes

Naming of Aromatic CompoundsWhen there is one group attached to a benzene ring then the compound is named by adding the group’s name to benzene.

chlorobenzene nitrobenzene(toluene)

methylbenzene1(chloromethyl)benzene2

1. There are many organic compounds that have older (non-systematic) names and these are sometimes used in text books.

2. Brackets are used in this case to emphasise that the chlorine atom is attached to the methyl group, not the benzene ring.

There are derivatives of benzene that have their own names.

benzoic acid benzaldehyde phenol

Cl NO2 CH3 CH2Cl

COOH CHO OH

Where there are more than one substituent on the carbon ring numbers are used to denote the position of the groups. The ‘parent’ group is placed at number one, and the numbering is done to get the lowest possible number. The groups are then named in alphabetical order.

X1

2

3

4

5

6

CH3

Cl

CH3

Cl

Br

Cl

2-chloromethylbenzene 3-chloromethylbenzene

1-bromo-2-chloromethylbenzene

CH3

NO2

NO2

1-methyl-2,4-dinitrobenzene84

11. Arenes

For benzene derivatives, such as benzoic acid, benzaldehyde and phenol, the existing substituent (-COOH, -CHO, -OH) is always number 1 with further substituents named in alphabetical order before the derivative’s name.

CHO

NO2

CH2CH3

COOH

ClCl

OH

NO2

NO2

O2N

4-ethyl-2-nitrobenzaldehyde

phenylethanoate

2,4,6-trinitrophenol

In some circumstances the benzene ring (C H -) becomes a substituent or a side chain. In these cases the term phenyl is used. One example is in esters.

6 5

OO

Reactions of Benzene – EA SrThe electron-rich delocalised bonds of benzene make it susceptible to attack by electrophiles. The stability of the delocalised electrons mean that reactions involve substitution of hydrogens, with the preservation of the delocalised bonds. The reactions are known as Electrophylic Aromatic Substitutions (EA S for short).r

H

H

H

H

H

H

X

H

H

H

H

H

3,5-dichlorobezioc acid

85

11. Arenes

Some examples of electrophilic substitution reactions, and the electrophiles involved.

NO2

Br

CH3

C

CH3O

NO2+

Br+

CH 3+

CH CO3+

+

+

+

+

+

+

+

+

H+

H+

H+

H+

EA S Mechanismr

This is a two-step reaction. The first step (rate-determining) is the reaction between the aromatic ring the electrophile to form a carbocation intermediate, followed by the loss of the leaving group (H ). +

E+

H

+

H E E

+ H+

E H E H E H E H

= +

The carbocation intermediate is made up of three resonance structures that delocalises the positive charge, imparting some stability.

+

+

+

86

11. Arenes

Ener

gy

Reaction

H

E H

+

Ea

E

E+

H+

Nitration

H SO + HNO H NO + HSO2 4 3 2 3 4

+ -

Nitration requires a mixture of concentrated nitric and concentrated sulfuric acid at moderate temperatures (ca. 50°C). Higher temperatures can give rise to multiple substitutions.

The reaction of sulfuric and nitric acid produce the nitronium (NO )ion which acts as the electrophile in the reaction.

2+

H NO NO + H O2 3 2

+

2+

87

11. Arenes

Chlorination and BrominationChlorine and bromine need the addition of a Lewis acid (electron acceptor) catalyst to produce electrophiles. Suitable catalysts are aluminium chloride, iron chloride or iron bromide.

Al Cl

Cl

Cl

Cl Cl

empty 3p orbital

Al

ClCl

Cl

Cl

Cl

-

+

Cl AlCl3

Cl

+ -

Drawn as:

Similar species are formed by the reaction of Cl with FeCl and between Br and FeBr .

2 3

32

Cl FeCl3

Cl

+ -Br FeBr3

Br

+ -

The halogen-Lewis acid complex transfers Cl to the benzene ring in the first step, followed by the release of a H from the ring. This then regenerates the Lewis acid catalyst (a similar mechanism is found for bromine).

Cl AlCl3

Cl

+ -

+

+

AlCl-4

Cl

HCl AlCl 3

Cl H

+

88

11. Arenes

SulfonationAromatic compounds such as benzene will react with concentrated sulfuric acid under reflux. The conditions produce the electrophile sulphur trioxide (SO3).

+

-

-

-

The reaction yields a sulfonic acid.

benzenesulfonic acid

Mechanism

89

11. Arenes

O R

H

Friedel-Crafts AlkylationFriedel-Crafts alkylation allow alkyl chains to be added to benzene rings. This uses haloalkanes and aluminium chloride as Lewis acid catalyst. The active species is similar to that involved in chlorination/bromination which then yields a carbocation that can act as an electrophile.

AlCl 3CH Cl3 + Cl AlCl3

CH

+ -

3

AlCl 4

-CH

3

++

CH3

+ CH3CH3 H

+H+

Friedel-Crafts AcylationFriedel-Crafts acylation takes place in a similar way to alkylation but instead of a haloalkane, an acyl chloride (RCOCl) is used as a precursor, together with aluminium chloride to yield a ketone.

R C

O

Cl

AlCl 3+ + AlCl 4-R O

+

+H

+

R OO R

+

90

11. Arenes

Activation/Deactivation, Directing EffectsThe electrophilic substitution reactions covered so far have been for unsubstituted benzene. These reactions can also be done with substituted benzene rings but the substituent can influence the reactivity of the benzene ring as well as the position of further substitutions.

In the case of methylbenzene (or other alkylbenzenes) the alkyl group can act as an electron donor and effectively ‘pushes’ electron density onto the ring, making it more susceptible to attack by electrophiles. The directing effect can be explained by looking at the resonance structures from each substitution.

NO2NO2

NO2

HNO3H SO

2 4

CH3CH3

NO2

CH3

NO2

HNO3H SO

2 4 +

NO2

HNO3H SO

2 4

Relative rates

1

25

0.0001

CH3H

NO2

CH3H

NO2

CH3H

NO2

CH3

H

NO2

CH3

H

NO2

CH3

H

NO2

CH3

H NO2

CH3

H NO2

CH3

H NO2

++ +

+ ++

++

+

Position 2

Position 3

Position 4

Substitution at 2 and 4 produces resonance structures with a positive charge next to the electron releasing alkyl group and so are stabilised.

91

11. Arenes

In the case of nitrobenzene, the nitro group is electron withdrawing so it removes electron density from the benzene ring, making it less reactive than benzene.

NO2

H

NO2

NO2

H

NO2

NO2

H

NO2

NO2H

NO2

NO2H

NO2

NO2

NO2

H

NO2

H NO2

NO2

H NO2

NO2

H NO2

++ +

+ ++

++

+

Position 2

Position 3

Position 4

Substitution at positions 2 and 4 place a positive charge next to the electron withdrawing nitro group which is unfavourable.

Some substituents that have lone pairs on the atom next to a carbon on the benzene ring are able to contribute even more electron density to the ring than alkyl groups so are even more reactive (and 2, 4 directing) than methyl benzene. Examples are phenol and analine.

OH NH2

anilinephenol

X X XX+ + +

-

-

-

92

11. Arenes

R NH2OHO R

NO2 Cl

Br

ActivatingDeactivating

2-, 4- directing3- directing3- directing

2-, 4- directing

93

11. Arenes

PolymersA polymer is a macromolecule (giant molecule) that is made up from repeating units. There are two main classes of polymers; addition polymers and condensation polymers.

Addition polymers usually involve one type of monomer (an alkene) where the 𝜋 bond is broken to make a new C-C 𝜎 bond. These reactions require a catalyst such as a mixture of TiCl4 and Al(C2H5)3.

n

n

Addition Polymers

C C

H

H

H

H

C C

H

H

H

H

C C

Cl

H

H

H

C C

Cl

H

H

H

C C

Me

H

H

H

C C

Me

H

H

H

C C

F

F

F

F

C C

F

F

F

F

C C

H

H

H

C C

H

H

H

Examples of addition polymers

n

n

n

n

n

n

n

n

n

n

ethene

chloroethene (vinyl chloride)

propene

tetrafluoroethene

phenylethene (styrene)

polyethene (polyethylene, PET)

polyvinylchloride (PVC)

polypropene (polypropylene, PP)

Teflon (PTFE)

polystyrene

94

12. Polymers

Addition Polymer StructuresThe physical properties of polymers are determined to a large extent by their chain structures.

For the simplest addition polymer, polyethene, there is only one structure possible.

H H

H H

H H

H H

H H

H H

H H

H H

For polymers made from substituted alkenes, such as propene there are several configurations of the polymer. These affect the way that the polymer chains can pack together.

C C

Me

H

H

H

H H

Me H

H H

Me H

H H

Me H

H H

Me H

H H

Me H

H H

Me H

H H

Me H

H H

Me H

Isotactic PolypropeneWhen the methyl groups are all on the same side of the chain the polymer chains can pack efficiently and maximise the Van der Waal’s forces between molecules. This produces a dense, high-melting, strong, heat resistant material which is strong both as solid and it fibres. It’s uses include plastic bags, food containers, pipes.

PolypropenePolypropene is manufactured by the polymerisation of propene using a mixture of TiCl4 and Al(C2H5)3 as a catalyst. The process produces largely stereoregular polymers that have their methyl groups either on the same side of the chain (isotactic) or on alternate sides (syntactic).

95

12. Polymers

H H

Me H

H H

H Me

H H

Me H

H H

H Me

H H

Me H

H H

H Me

H H

Me H

H H

H Me

Syndiotactic PolypropeneSyndiotactic polypropylene has the methyl groups at alternate sides of the chain. This allows close packing of chain, however not as close as in isotactic polypropylene. This gives a dense strong polymer that is more flexible than isotactic polypropylene. This is used in specialist applications such a medical tubing.

Atactic PolypropeneWhen the methyl groups are randomly arranged along the polymer chain the chain packs badly and the result is a soft and elastic polymer which has limited use but it is used in sealant. or formulated into adhesives.

H H

Me H

H H

Me H

H H

H Me

H H

Me H

H H

H Me

H H

H Me

H H

Me H

H H

Me H

96

12. Polymers

Condensation polymers are formed when monomers react with each other to eliminate a smaller molecule (for example H2O). Natural polymers, such as polypeptides are formed this way as are many synthetics. Monomers suitable for condensation polymerisation contain groups such as carboxylic acids (-COOH), amines (-NH2) and alcohols (-OH). Frequently two different monomers are used.

Condensation Polymers

C

O

OH

C

O

OH

OH OH C

O

O C

O

O H2O+ +

ester linkages

C

O

N

H

C

O

N

H

C

O

OH

C

O

OH

+ H2O+NH2 NH2

amide linkages

C C

O

OH O

OH

C COH OH

H

H

H

H

+ CC

OO

O C C O

H H

H H

H2O+

PolyesterA polyester is a polymer that is formed between a dicarboxylic acid(for example benzene-1,4-dicarboxylic acid) and a diol such as ethan-1,2-diol. Polyesters are used as fibres as well as for the manufacture plastic bottles.

benzene-1,4-dicarboxylic acid ethan-1,2-diol poly(ethylene terephthalate) PET

NylonNylon is a polyamide that is prepared from a dicarboxylic acid and a diamine. The polymer can be processed into fibres, films and shapes.

C CH2 C

O

OH

O

OH

CH2 NH2NH2 C CH2 C

O O

NH CH2 NH H2O++( )4 ( )6( )4

97

12. Polymers

( )6

KevlarKevlar is a polyamide which is considerably stiffer than nylon due to the rigidity imparted by the inflexible aromatic rings in the chain.

NHC

O

C

O

NH

Condensation Polymers – Biodegradability Because condensation polymers contain functional groups they are far more reactive than addition polymers. Polymers can be made that will break down naturally after their use through hydrolysis reactions. An example is polyurethane.

NHC

O

CH2 NH C

O

O CH2 CH2 O

BiopolymersAmino acids are able to join together when the amine group of one reacts with the carboxylic acid of the other. The bond formed (identical to an amide) is called a peptide link (-CONH-). Two amino acids form a dipeptide, three a tripeptide and so on. When the number increases (10 amino acids or so) it is called a polypeptide. Polypeptides make up protein chains.

O

NH2

OH

O

NH2OH

O

NH2

NH

O

OHH2O++

alanine glycine

98

12. Polymers

Chemical Tests for functional GroupsSimple chemical tests can be carried out to distinguish between classes of compound.

Alkenes

Alkenes can be identified by treating a sample with bromine. If an alkene is present then the bromine water (orange) will decolourise.

+ Br2

Positive result (alkene present)

Negative result (alkene absent)

Carboxylic Acids

Carboxylic acids can be identified by adding a sample to an aqueous solution of sodium carbonate (Na2CO3) or sodium hydrogen carbonate (NaHCO3). If an acid is present CO2 will be released and effervescence (bubbling) will be seen. The acid will be converted to it’s sodium salt.

Positive result (acid present)

Negative result (acid absent)

+ Na2CO32 2 + CO2 + H2O

99

13. Tests for functional groups

Distinguishing between Tertiary and Primary/Secondary Alcohols

Primary and secondary alcohols can be readily oxidised to aldehydes or ketones respectively with acidified potassium dichromate(VI) (K2Cr2O7) solution, whereas tertiary alcohols will remain unchanged.

Primary/secondary alcohol

Tertiary alcohol

H+/K2Cr2O7

H+/K2Cr2O7

100


Distinguishing between Aldehydes and Ketones

The tests that distinguish between aldehydes and ketones rely on the fact that aldehydes are readily oxidised and ketones much less so. A range of oxidising agents are able to oxidise aldehydes to carboxylic acids. Ketones are only oxidised by strong oxidising agents.

+ [O]

1. Acidified potassium dichromate(VI) (K2Cr2O7) solution

Aldehyde

Ketone

2. Tollens' reagent (‘The Silver Mirror Test) - [Ag(NH3)2]+.

Aldehyde

Ketone

‘Silver mirror’ or grey precipitate

3. Fehling’s Solution - complexed copper(II) ions in an alkaline solution. Sample added and mixture heated on a water bath.

Aldehyde

Ketone

Red precipitate

101


An Introduction to Organic Chemistry - Canvas

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